Printing ink

ABSTRACT

The present invention provides an inkjet ink comprising 5 to 30% by weight of organic solvent based on the total weight of the ink, a radiation curable monofunctional monomer, a radiation curable multifunctional monomer, a photoinitiator and a colourant.

This invention relates to a printing ink and in particular to an ink foruse in inkjet printers.

Inkjet printing is becoming an increasing popular technique for theproduction of graphic images for advertising and the like. An inkjetprinter comprises a printhead that includes a series of nozzles whicheject ink onto a substrate. Two main ink chemistries are used in theart: inks that dry by exposure to ultraviolet radiation and inks thatdry by solvent evaporation.

Inks that dry by exposure to UV radiation comprise a UV curable carrierthat polymerises, or crosslinks, on exposure to UV light to form acrosslinked polymer film. These inks dry quickly, adhere to a wide rangeof substrates, and provide printed films with good solvent resistance.However, UV curable inks have relatively high viscosities and aretherefore generally heated at the printhead in order to reduce theviscosity of the ink sufficiently so that the ink can be jetted. Forexample, a UV curable ink that has a viscosity of 20 to 22 mPas at 25°C. would be heated to around 38 to 45° C. at the printhead in order toreduce the viscosity to less than 10 mPas. This requirement for heatingadds complexity to the printhead which means that currently availableprintheads for use with UV curable inks are relatively expensive andmore complex engineering is required to achieve the desired reliability.Printheads that are adapted to print this type of ink include theDimatix SE128 and the Toshiba Tec CA4 drop-on-demand piezo print heads.

Because of their intrinsically low viscosity, solvent-based inkjet inksdo not need to be heated in order to be jetted satisfactorily at theinkjet printhead; indeed some lower cost printheads simply cannot beheated. The printheads used to print solvent-based inkjet inks aretherefore typically lower cost, such as the drop-on-demand piezo printheads made by Epson. However, current solvent-based inkjet inks may notadhere to certain types of substrate, particularly non-porous substratessuch as plastics, and the printed films have poor resistance tosolvents.

There therefore exists a need for an inkjet ink that has good solventresistance and that can be printed on a wide range of substrates, butwhich can be printed without the need for heating at the printhead andcan therefore be used with currently available, low cost printheads.

Accordingly, the present invention provides an inkjet ink comprising 5to 30% by weight of organic solvent-based on the total weight of theink, a radiation curable monofunctional monomer, a radiation curablemultifunctional monomer, a photoinitiator and a colourant.

The inks of the present invention preferably have a viscosity of 15 mPasor less at 25° C., preferably 10 mPas or less and most preferably 7 mPasor less. Viscosity can be measured using a Brookfield DV1 low-viscosityviscometer running at 20 rpm at 25° C. with spindle S00.

The presence of an organic solvent in the ink of the present inventionreduces the viscosity of the ink, which allows the ink to be jetted froma printhead operating at ambient temperature. This means that the inksof the present invention can be jetted using a wider range of currentlyavailable printheads such as those that are typically used for printingsolvent-based inkjet inks Examples of such printheads includes the

Epson drop-on-demand piezo print heads. However, the inks of theinvention surprisingly maintain the advantageous properties of UVcurable inkjet inks such as good adhesion to a wide range of substratesand good solvent resistance in the printed film.

The presence of an organic solvent in the inks of the invention is alsoexpected to provide the printed images with improved flexibility andhigher gloss when compared to known UV curable inks.

The inks of the present invention include an organic solvent. Theorganic solvent is in the form of a liquid at ambient temperatures andis capable of acting as a carrier for the remaining components of theink. The organic solvent component of the inks of the invention may be asingle solvent or a mixture of two or more solvents. As with knownsolvent-based inkjet inks, the organic solvent used in the ink of thepresent is required to evaporate from the printed ink, typically onheating, in order to allow the ink to dry.

The organic solvent may be selected from any solvent that is typicallyused in the printing industry, such as glycol ethers, glycol etheresters, alcohols, ketones, esters and mixtures thereof. Other suitablesolvents include organic carbonates.

Solvents having a boiling point of greater than 100° C. are preferred.Examples include 1-methoxy-2-propanol, diethylene glycol diethyl ether,2-butoxyethylacetate and propylene carbonate. The use of a lowvolatility solvent or solvent mixture minimises evaporation from ink inthe inkjet nozzle when the printer is inactive, which prevents theformation of encrusted deposits that can lead to blocked or deviatednozzles.

The solvent is present in the ink in an amount of 5 to 30% by weight,based on the total weight of the ink, preferably 7 to 25%, morepreferably 11 to 20%.

In one embodiment the organic solvent is a low toxicity and/or a lowodour solvent. Solvents that have been given VOC exempt status by theUnited States Environmental Protection Agency or European Council arepreferred.

Other solvents may be included in the organic solvent component. Forexample, solvents such as 2-ethylhexyl acetate may be introduced as partof commercially available pigment dispersions. Solvent may also beintroduced with commercially available polymeric dispersants such as Byk168.

The ink of the present invention is preferably substantially free ofwater, although some water will typically be absorbed by the ink fromthe air or be present as impurities in the components of the inks, andsuch levels are tolerated. For example, the ink may comprise less than5% by weight of water, more preferably less than 2% by weight of waterand most preferably less than 1% by weight of water, based on the totalweight of the ink.

Any radiation curable mono- and multifunctional monomers that aresuitable for use in an inkjet ink may be used in the ink of theinvention and such monomers are well known in the art. The ink comprisesone or more monofunctional monomers and one or more multifunctionalmonomers.

The radiation curable monomers may polymerise by cationic polymerisationin which cases the mono- and multifunctional monomers can be selectedfrom epoxides, allyl ethers, vinyl ethers, oxetanes andhydroxy-containing compounds, for example.

Preferably, however, the mono- and multifunctional monomers polymeriseby free radical polymerisation.

Suitable free radical polymerising monofunctional monomers includemonofunctional (meth)acrylates, N-vinyl amides, N-acryloylamines,α,β-unsaturated ether monomers and mixtures thereof.

Monofunctional (meth)acrylate monomers are well known in the art.Preferably the monofunctional (meth)acrylate monomers are esters ofacrylic acid. Preferred examples include phenoxyethyl acrylate (PEA),cyclic TMP formal acrylate (CTFA), isophoryl acrylate, isobornylacrylate (IBOA), tetrahydrofurfuryl acrylate (THFA),2-(2-ethoxyethoxy)ethyl acrylate, octyl-decyl acrylate (ODA), tridecylacrylate (TDA), isodecyl acrylate (IDA) and lauryl acrylate. PEA,2-(2-ethoxyethoxy)ethyl acrylate and mixtures thereof are particularlypreferred.

N-vinyl amides and N-(meth)acryloyl amines may also be used in the inksof the invention. N-vinyl amides are well-known monomers in the art anda detailed description is therefore not required. N-vinyl amides have avinyl group attached to the nitrogen atom of an amide which may befurther substituted in an analogous manner to well known (meth)acrylatemonomers. Preferred examples are N-vinyl caprolactam (NVC) and N-vinylpyrrolidone (NVP). Similarly, N-acryloyl amines are also well-known inthe art. N-acryloyl amines also have a vinyl group attached to an amidebut via the carbonyl carbon atom and again may be further substituted inan analogous manner to known (meth)acrylate monomers. A preferredexample is N-acryloylmorpholine (ACMO).

Suitable α,β-unsaturated ether monomers include vinyl ethers such asethylene glycol monovinyl ether.

The total amount of radiation curable monofunctional monomer ispreferably 20 to 80% by weight, more preferably 30 to 70%, based on thetotal weight of the ink.

Preferred multifunctional monomers are (meth)acrylate monomers andα,β-unsaturated ether monomers. Di-, tri- and tetra- functional monomersare preferred. Examples of the multifunctional acrylate monomers thatmay be included in the ink-jet inks include hexanediol diacrylate,trimethylolpropane triacrylate, pentaerythritol triacrylate,polyethyleneglycol diacrylate (for example tetraethyleneglycoldiacrylate), dipropyleneglycol diacrylate, tri(propylene glycol)triacrylate, neopentylglycol diacrylate, bis(pentaerythritol)hexaacrylate, and the acrylate esters of ethoxylated or propoxylatedglycols and polyols, for example, propoxylated neopentyl glycoldiacrylate, ethoxylated trimethylolpropane triacrylate, and mixturesthereof. Particularly preferred are di- and trifunctional acrylates.

In addition, suitable multifunctional monomers include esters ofmethacrylic acid (i.e. methacrylates), such as hexanedioldimethacrylate, trimethylolpropane trimethacrylate, triethyleneglycoldimethacrylate, diethyleneglycol dimethacrylate, ethyleneglycoldimethacrylate, 1,4-butanediol dimethacrylate. Mixtures of(meth)acrylates may also be used.

Suitable α,β-unsaturated ether monomers include vinyl ethers. Examplesare well known in the art and include triethylene glycol divinyl ether,diethylene glycol divinyl ether and 1,4-cyclohexanedimethanol divinylether. Mixtures of α,β-unsaturated ether monomers may be used.

The presence of a multifunctional monomer in the ink of the invention isbelieved to provide the final printed film with superior solventresistance and toughness.

The total amount of radiation curable multifunctional monomer ispreferably 5 to 60% by weight, more preferably 10 to 50% by weight, morepreferably 15 to 40%, most preferably 20 to 40%, based on the totalweight of the ink.

In one embodiment of the invention, the ink comprises at least oneα,β-unsaturated ether monomer and at least one (meth)acrylate monomer.

(Meth)acrylate is intended herein to have its standard meaning, i.e.acrylate and/or methacrylate. Mono- and multifunctional are alsointended to have their standard meanings, i.e. one and two or moregroups, respectively, which take part in the polymerisation reaction oncuring.

The radiation curable monomers used in the ink of the inventionpreferably have molecular weight of 450 or less. In a preferredembodiment, the ink of the invention comprises less than 10% by weight,more preferably less than 5% by weight of radiation curable materialthat has a molecular weight of greater than 450, based on the totalweight of the ink. Most preferably, the ink is substantially free ofradiation curable material that has a molecular weight of greater than450.

Furthermore, preferred inks of the invention are substantially free ofnon reactive, or passive, resin binder. By “non reactive, or passive,resin binder” is meant a resin material that does not include reactivegroups that are able to crosslink on exposure to radiation. In otherwords, non reactive or passive resin binders are not radiation curablematerials. Resin binders typically have a molecular weight of10,000-100,000 (as determined by GPC with polystyrene standards) and areused in known solvent-based inks to improve the solvent resistance ofthe printed film. The inks of the invention include radiation curablematerial which is able to crosslink in order to provide a solventresistant film, however, and a resin binder is not therefore required.The absence of a relatively high molecular weight resin binder in thepreferred inks of the invention means that very low viscosity inks canbe provided.

By “substantially free of” is meant that only trace amounts of materialmay be present. These trace amounts may be introduced into the ink forexample as impurities in commercially available radiation curablemonomers, or components in commercially available pigment dispersions.

The ink of the invention includes one or more photoinitiators. When theink of the invention includes a free radical polymerisable material thephotoinitiator system includes a free radical photoinitiator and whenthe ink includes a cationic polymerisable material the photoinitiatorsystem includes a cationic photoinitiator.

The free radical photoinitiator can be selected from any of those knownin the art. For example, benzophenone, 1-hydroxycyclohexyl phenylketone,1-[4-(2-Hydroxyethoxy)-phenyl]-2-hydroxy-2-methyl-1-propane-1-one,2-benzyl-2-dimethylamino-(4-morpholinophenyl)butan-1-one, isopropylthioxanthone, benzil dimethylketal, phenyl bis(2,4,6-trimethyl benzoyl)phosphine oxide, (2,4,6-trimethylbenzoyl) diphenylphosphine oxide ormixtures thereof. Such photoinitiators are known and commerciallyavailable such as, for example, under the trade names Irgacure andDarocur (from Ciba) and Lucerin (from BASF).

In the case of a cationically curable system, any suitable cationicinitiator can be used, for example sulphonium or iodonium based systems.Non limiting examples include Rhodorsil PI 2074 from Rhodia; MC AA, MCBB, MC CC, MC CC PF, MC SD from Siber Hegner; UV9380c from AlfaChemicals; Uvacure 1590 from UCB Chemicals; and Esacure 1064 fromLamberti spa.

Preferably the photoinitiator is present in an amount of 1 to 20% byweight, preferably 4 to 10% by weight, based on the total weight of theink.

The ink comprises at least one colourant. The colourant may be eitherdissolved or dispersed in the liquid medium of the ink. Preferably thecolourant is a dispersible pigment, of the types known in the art andcommercially available such as under the trade-names Paliotol (availablefrom BASF plc), Cinquasia, Irgalite (both available from Ciba SpecialityChemicals) and Hostaperm (available from Clariant UK). The pigment maybe of any desired colour such as, for example, Pigment Yellow 13,Pigment Yellow 83, Pigment Red 9, Pigment Red 184, Pigment Blue 15:3,Pigment Green 7, Pigment Violet 19, Pigment Black 7. Especially usefulare black and the colours required for trichromatic process printing.Mixtures of pigments may be used.

Pigment particles dispersed in the ink should be sufficiently small toallow the ink to pass through an inkjet nozzle, typically having aparticle size less than 8 μm, preferably less than 5 μm, more preferablyless than 1 μm and particularly preferably less than 0.5 μm.

Commercially available pigment dispersions tend to comprise around 40 to50% by weight of solvent or carrier based on the total weight of thepigment dispersion and the pigment dispersion typically makes up around2 to 20% by weight of the ink and sometimes more.

The colourant is preferably present in an amount of 1 to 10% by weight,based on the total weight of the ink.

Other components of types known in the art may be present in the ink toimprove the properties or performance. These components may be, forexample, surfactants, defoamers, dispersants, synergists for thephotoinitiator, stabilisers against deterioration by heat or light,reodorants, flow or slip aids, biocides and identifying tracers.

The ink of the present invention can be printed using any inkjet printerthat is suitable for printing solvent-based ink jet inks. A source ofradiation is required in order to cure the radiation curable monomers,however.

Ink-jet printers comprise a printhead that includes a series of nozzlesthrough which ink is ejected onto a substrate. The printhead istypically provided on a carriage that traverses the print width (movesback and forth across the substrate) during the printing process.

Preferably, the ink of the invention is jetted on to a substrate using aprinthead for solvent-based inkjet inks, such as an Epson drop-on-demandpiezo printhead. Printheads for solvent-based inks do not include ameans for heating the ink in the printhead. In this embodiment, the inkis not heated at the printhead and therefore remains at ambienttemperature while in the printhead and during jetting.

The ink of the invention is preferably jetted at less than 35° C., morepreferably less than 30° C., and most preferably at about 25° C.

The ink of the present invention dries by a combination of evaporationof the organic solvent, and curing of the radiation curable monomersupon exposure to actinic radiation. The evaporation and the curing stepscan take place in any sequence including simultaneously.

The solvent is evaporated from the printed ink using known methods,preferably by heating the printed ink. Heat may be applied through thesubstrate and/or from above the substrate, for example by the use ofheated plates provided under the substrate or radiant heaters providedabove the substrate. In one example, the ink is jetted onto a preheatedsubstrate that then moves over a heated platen.

The radiation curable monomers that are included in the ink of theinvention are cured by exposing the ink to actinic radiation, preferablyUV radiation. Sources of UV radiation are known to the person skilled inthe art and include mercury discharge lamps, flash lamps and lightemitting diodes (LEDs). One or more radiation sources may be used. TheUV source may be a static lamp, or the source can placed on a carriagethat allows the source to move across the print width, for example. Thesource can be provided on the same carriage as the printhead, or can beprovided on a separate carriage.

When a mercury discharge lamp is used as a UV source, typically noseparate solvent evaporation step is required because evaporation of theorganic solvent provided in the ink can be achieved by exposure to theinfra red portion of the radiation that is emitted from the mercurydischarge lamp.

In one embodiment of the invention, the radiation curable material iscured in two steps. In the first step, the radiation curable material ispartially cured in order to “pin” the ink to the substrate. This partialcure can be achieved, for example, by providing a source of UV radiationsuch as an LED array on the carriage that moves the printhead back andforth across the substrate. It is understood that the surface of theprinted ink is still able to flow to some extent after partial cure,which may contribute to improved gloss in the final printed image. Theink is then exposed to a further dose of radiation in order tosubstantially complete the cure and solidify the ink. This second curestep can be achieved using a second radiation source, such as a staticradiation source that is positioned downstream from the printercarriage. In this embodiment of the invention, the solvent evaporationstep can take place before the pinning step, between the pinning stepand full cure step, or after the full cure step. Alternatively, thesolvent evaporation can take place simultaneously with one of the curingsteps.

The nature of the substrate is not limited and includes any substratewhich may be subjected to inkjet printing. However, the inks of thepresent invention are particularly suited for printing onto substratessuch as styrene, PolyCarb (a polycarbonate), BannerPVC (a PVC) and VIVAK(a polyethylene terephthalate glycol modified).

The present invention also provides a method of inkjet printing asdescribed above, and a substrate having the printed ink thereon. The inkof the present invention is particularly suited to piezoelectricdrop-on-demand inkjet printing.

The present invention further provides a printing apparatus and aninkjet ink cartridge containing an inkjet ink as defined herein. Thecartridges comprise an ink container and an ink delivery port which issuitable for connection with an inkjet printer.

The inks of the present invention may be prepared by known methods suchas stirring with a high-speed water-cooled stirrer, or milling on ahorizontal bead-mill.

The invention will now be described with reference to the followingexamples, which are not intended to be limiting.

EXAMPLES

Cyan and black inkjet ink formulations according to the invention(Examples 1 and 2) having the compositions shown in Table 1 wereprepared by mixing the components in the given amounts. Amounts aregiven as weight percentages based on the total weight of the ink.

The inks were drawn down onto semi-rigid 220 μm PVC using a no 2 Kbar,depositing a wet film weight of 12 microns. The ink of Example 1 wascured by exposing the printed film to a medium pressure mercury lampproviding a UV dose of 400 mJ/cm². The ink of Example 2 was cured byexposing the printed film to a medium pressure mercury lamp providing aUV dose of 600 mJ/cm².

The relative solvent resistance of the cured prints was assessed byrubbing with a soft cloth soaked in isopropyl alcohol. The number ofdouble rubs required to break through to the substrate was recorded ineach case (100 maximum).

The adhesion of the dried prints to the substrate was tested by scoringthe print with a cross-hatch tester to produce a number of squares. 3Mpressure sensitive Scotch-tape was applied to the scored film and thenremoved. The number of squares of ink removed with the tape was countedand the level of adhesion was rated 1 (poor) to 5 (good).

The results are shown in Table 2.

It can be seen that the inks of the invention have significantly lowerviscosities than currently available UV curable inks, that typicallyhave viscosities of 20 to 30 mPas at 25° C., but the inks of theinvention maintain the advantageous properties of UV curable inks suchas adhesion to non-porous substrates, for example PVC, and good solventresistance of the printed film.

TABLE 1 Example 1 Example 2 Phenoxy ethyl 17.8 16.3 acrylate N-vinyl14.5 13.4 caprolactam Carbitol 8.4 7.9 acrylate Hexane diol 33.8 32diacylate Cyan pigment 3.8 — dispersion Black pigment — 4.4 dispersionIrgacure 819 3.4 3.2 Irgacure 2959 1.7 1.6 Irgacure 369 0.9 0.81-methoxy 15 20 2-propanol Stabiliser ST-1 0.7 — TegoRad 2100 — 0.4Total 100 100 Viscosity at 5.6 5.5 25° C./mPas

TABLE 2 Example 1 Example 2 Solvent 100+ 100+ resistance Adhesion  5  5to PVC

1. An inkjet ink comprising 7 to 30% by weight of organic solvent, basedon the total weight of the ink, a radiation curable monofunctionalmonomer, a radiation curable multifunctional monomer, a photoinitiatorand a colourant, wherein the ink has a viscosity of 7 mPas or less at25° C.
 2. (canceled)
 3. The ink according to claim 1 wherein the organicsolvent is selected from glycol ethers, glycol ether esters, alcohols,ketones, esters, organic carbonates and mixtures thereof.
 4. The inkaccording to claim 1 wherein the organic solvent has a boiling point ofgreater than 100° C.
 5. The ink according to claim 1 wherein the organicsolvent is present in an amount of 7 to 25% by weight based on the totalweight of the ink.
 6. The ink according to claim 1 comprising less than5% by weight of water, preferably less than 2% by weight of water andmore preferably less than 1% by weight of water, based on the totalweight of the ink.
 7. The ink according to claim 1 wherein the radiationcurable monomers are capable of polymerising by radical polymerisation.8. The ink according to claim 7 wherein the radiation curablemonofunctional monomer is selected from monofunctional (meth)acrylates,N-vinyl amides, N-acryloylamines, α,β-unsaturated ethers and mixturesthereof.
 9. The ink according to claim 1 wherein the radiation curablemonofunctional monomer is present in an amount of 20 to 80% by weight,preferably 30 to 70%, based on the total weight of the ink.
 10. The inkaccording to claim 1 wherein the radiation curable multifunctionalmonomer is present in an amount of 5 to 60% by weight, preferably 10 to50% by weight, more preferably 20 to 40%, based on the total weight ofthe ink.
 11. The ink according to claim 1 wherein the radiation curablemonomers have a molecular weight of 450 or less.
 12. The ink accordingto claim 1 that is substantially free of radiation curable material thathas a molecular weight of greater than
 450. 13. The ink according toclaim 1 that is substantially free of resin binder.
 14. A method ofinkjet printing comprising printing the inkjet ink as claimed in claim 1on to a substrate and drying the ink by exposing the printed ink to heatand actinic radiation.
 15. The method according to claim 14 wherein theink is jetted at less than 35° C., preferably less than 30° C. and mostpreferably at about 25° C.
 16. The method according to claim 14 whereinthe ink is jetted at ambient temperature.
 17. The method according toclaim 14 wherein the source of actinic radiation is a mercury dischargelamp and exposure to the mercury discharge lamp causes simultaneousevaporation of the organic solvent and curing of the radiation curablemonomers.
 18. An inkjet printing apparatus comprising a printhead forprinting a solvent-based inkjet ink, a heater and a source of actinicradiation.
 19. The inkjet printing apparatus according to claim 18wherein the printhead does not include a means for heating ink in theprinthead.
 20. An inkjet cartridge comprising the ink as claimed inclaim
 1. 21. A method of using a printing apparatus as claimed in claim18 in the method of claim
 14. 22. A substrate having the ink as claimedin claim 1 printed thereon.